The present invention relates to a load responsive control valve for generating an output pressure corresponding to a vehicle load.
For example, in existing vehicles having air springs, a load responsive valve device, may be employed to control the braking force corresponding to changes in vehicle weight due to loading and unloading of passengers, etc. As shown in FIG. 1, a known valve device of this type consists of a chamber 1 to which air spring pressure AS.sub.1 corresponding to the vehicle load is connected, a chamber 3 to which fluid pressure is connected from an air source via an inlet 2, and a chamber 5 from which the controlled output pressure is discharged through an output 4. A diaphragm piston 6 is arranged in chamber 1 for responding to the air spring pressure AS.sub.1, and a spring 7 is placed against diaphragm piston 6. This spring 7 is set at a suitable braking pressure for an empty vehicle. In chamber 5, there is provided an inlet-outlet valve 8 to which is connected a diaphragm piston 9 subject opposingly to the spring 7 and the output pressure in chamber 5. A valve element 10 of inlet-outlet valve 8 controls fluid pressure communication at a passage opening 5a connecting chamber 3 and chamber 5 in response to up and down movement of inlet-outlet valve 8.
When the air spring pressure AS.sub.1 is lower than a value corresponding to the preset tension of spring 7 representing the desired braking pressure for an empty vehicle, the inlet-outlet valve 8 is moved upward by the force of spring 7 and valve element 10 is opened allowing the flow of pressurized air from the chamber 3 to the chamber 5. When the downward force exerted on the diaphragm piston 9 by the pressure in the chamber 5 is equalized with the upward force of spring 7, the inlet-outlet valve 8 is moved downward, and the passage opening 5a is closed by the air inlet valve 10. Thus, the output pressure discharged from outlet 4 is controlled to maintain a pressure corresponding to the preset value of spring 7, that is, the braking pressure of the empty vehicle.
When the air spring pressure AS.sub.1 is higher than the empty vehicle pressure, diaphragm piston 6 is moved upward against the tension of spring 7, which in turn moves the inlet-outlet valve 8 and valve 10 upward, thereby opening valve 10 to establish fluid pressure communication between chambers 3 and 5 via opening 5a. When the upward force of spring 7 corresponding to the braking pressure of the empty vehicle and the force obtained by subtracting the force of the spring 7 from the force exerted on the diaphragm piston 6 by the air spring pressure AS.sub.1 is balanced by the downward force of pressure in chamber 5 acting on diaphragm piston 9, valve 10 is closed to terminate further buildup of output pressure. The relationship is as shown in FIG. 5.
In the case of a load response control valve device of the above-mentioned type, a loss of air spring pressure AS.sub.1 due to damage to the air spring or diaphragm piston 6 results in an empty car braking force equivalent to that of spring 7, even in the case of a loaded vehicle. This results in an undesirable increase in the distance required for stopping the vehicle and may result in accidents, such as collisions with the vehicle stopped in front of the said vehicle, thus posing obstacles to safe driving.